Conventional pearlescent pigments are based on products including the natural product mica, which constitutes a transparent, low-index, platelet-shaped substrate having a slightly yellowish intrinsic color. These substrates are coated with high-index oxides such as, preferably, TiO2 and/or Fe2O3, resulting in the known pearl luster effects. In addition to its inherent color, mica has the disadvantages of being subject, as a natural product, to large variations, and possessing an inconsistent thickness, too many steps within one substrate, and an inconsistent size. Many attempts have been made, therefore, to replace mica with synthetic materials which can be varied and can be controlled more effectively in terms of their dimensions (thickness, thickness distribution, size) and optical properties (transparency, deliberate coloring).
EP 0 240 952 A2 describes a method of producing platelet-shaped material by applying liquid metal compounds or their solutions or sols to a continuous belt to form a thin film. This film is subsequently solidified and converted into platelet-shaped material.
WO 93/08237 likewise describes substrates composed of metal oxides, but, additionally, colorants have been introduced into the substrates. The metal oxides are formed by applying a corresponding precursor in the form of a thin film to a continuous belt. The solidified film that results from drying is treated with acid, coated, washed, and then separated from the support medium. Coating at least one side of the film with semitransparent metal layers or high-index metal oxides generates the conditions required in optical terms for effect pigments.
Pearlescent pigments based on a platelet-shaped substrate of aluminum oxide are described in EP 0 763 573 A2.
Al2O3-based pearlescent pigments in particular, with regard to their mechanical properties, are very hard and brittle. Pigments of this kind may cause damage, for example, to circuit lines in automotive finishing systems.
Pearlescent pigments having a substrate composed of C-glass are described in EP 0 912 640 E1.
Furthermore, effect pigments based on purely organic substrates are known from WO 01/92394 A1. With this kind of effect pigment, the adhesion of the metal oxide layers to the purely organic substrate is poor.
DE 10 2004 032 799 A1 describes effect pigments of uniform size and shape. The platelet-shaped substrates possess a circular or elliptical shape or else a polygonal shape and are composed of the metal oxides already known.
DE 196 38 797 A1 likewise describes effect pigments of uniform size and shape. In this case, however, the substrates are composed of organic, polymerizable materials. The compounds which can be incorporated by copolymerization include liquid-crystalline compounds. The polymerization is carried out in a sheetlike structure having continuous openings of defined shape and size, such as, for example, meshes made of plastic or of metal, the mesh size defining the geometry of the pigments produced.
A disadvantage effecting some conventional effect pigments is their great hardness. Particularly when using pearlescent pigments based on Al2O3 platelet substrates, the pumped circulation of paints or varnishes in pipeline systems, or application via spraying nozzles, may be accompanied by damage to the pipelines or spray nozzles, through abrasion.
A disadvantage effecting effect pigments based on organic-chemical substrates is that layers of metal oxide applied to them are readily detachable.